Among 39 patients with normal echocardiograms measurement of the subaortic diameter in the parasternal long axis cross sectional echocardiographic images no differences in left ventricular end-diastolic dimension or left atrial size between children and adults was observed when these two dimensions were normalized by dividing by the subaortic diameter. Thus the constant proportions of left ventricular end-diastolic dimension/subaortic diameter and left atrial size/subaortic diameter in the normal heart and the usual lack of variance of the subaortic diameter in disease make this dimension a convenient and effective parameter for normalized left ventricular end-diastolic dimension and left atrial size.